Electronic telephone system featuring switching networks having thyristors for single-wire switching

ABSTRACT

An electronic telephone switching system is described having peripheral units including a plurality of subscriber and line circuits, a plurality of trunk and tie line circuits, at least one attendant line circuit, a plurality of signaling tone receivers, and a plurality of link control circuits. Switching is carried out by a one wire speech path switching matrix and a one wire tone switching matrix. The system is centrally controlled and includes a central processing unit, a program memory, a scratch pad memory and a customer memory. Information, commands and addresses are communicated between the peripheral units and the central control and within the central control via bus lines. The bus lines between the peripheral units and the central control transmit data with lower rise time pulses than the bus lines interconnecting the components of the central control. Each electronic crosspoint switch has a two terminal conducting path and a control terminal for controlling the state of conduction of the conducting path. Different conducting path terminals of the speech path matrix and tone path matrix crosspoint switches are operated by the same peripheral link unit. The matrix coordinates to be operated are defined by polling the different peripheral units connected to the coordinates in question.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to applications filed on even date herewith,Ser. Nos. 773,309; 773,310; 773,311; 773,313; 773,314 and 773,315.

BACKGROUND OF THE INVENTION

The invention relates to an electronic telephone switching system havingperipheral units such as a plurality of subscriber and line circuits, aplurality of trunk and tie line circuits, at least one attendant linecircuit, a plurality of signaling tone receivers, and a plurality oflink control circuits. The system includes a one wire switching speechpath matrix, a one wire switching tone matrix, and a system clock unit,including a common control comprising a program memory, a scratch padmemory, a customer memory and a central processing unit and furtherincluding, as well, incoming and outgoing information and commands asaddresses between the different units of the common control unit and thedifferent peripheral units information carrying bus lines, whereby saidbus lines for changing of information in the common control carryinformation with faster rise time pulses than the peripheral units withthe common control unit, wherefore a peripheral interface logic dividesthe faster bus lines from the slower peripheral bus lines.

U.S. Pat. Nos. 3,904,831 and 3,943,297 describe a private automaticbranch exchange (PABX) wherein subscriber line circuits signalingreceivers, signaling transmitters and connecting sets of attendantstations or consoles and other special junctors are connected as inputand output units to the horizontal lines of a switching matrix. Feedingsets, internal junctors or internal feeding sets, as the case may be,are connected to the vertical lines of the switching matrix. Aconnection between subscriber stations connected to the rows and otherinput and output units is established by operating the crosspoints attwo matrix crossings. As an example, the connection of a subscriberstation to a junctor circuit is established by closing the crosspointswitch at only one crosspoint of the switching matrix. The number ofcrosspoints is dependent on the number of wires to be switched.

Techniques for establishing connections between input/output (I/O)devices connected to the horizontal lines of the switching matrix byusing a switching matrix having one or more stages and bythrough-connecting more than one crosspoint are exemplified by U.S. Pat.No. 3,308,242 and British Pat. No. 1,058,893. The latter shows that thisform of connection may also be established by wires of a singleswitching stage of the switching matrix, each of which connects at leasttwo crosspoints. In this regard, reference is made to the article, "Lenouveau system telephonique Trachsel-Gfeller a reflecteurs crossbars" inthe Swiss publication TECHNIQUE PTT, 1955, No. 3, pp. 115-129, and to"Der X/53er-Schweizer Kreuzschienen Hausautomat" published inHASLER-MITTEILUNGER OF HASLER AG, Bern, No. 3, 1957, pp. 57-67, as wellas to U.S. Pat. No. 2,955,165. Various types of systems are employed inthese known telephone systems, i.e., both directly controlled systemsand those using common-control equipment.

In some of the aforementioned facilities, subscriber stations, as wellas junctor and feeding circuits are to be scanned in successive cyclesin accordance with the last-look principle. For example, it is knownfrom U.S. Pat. Nos. 3,904,831 and 3,943,297, when a subscriberoriginates a call which is to be identified during the scanning cycle,after assignment of a junctor and feeding circuit, to commence thefurther states of operation, such as transmission of the selectionsignals, during the scanning cycle of the relevant junctor and feedingcircuit in use. This means that the scan period of a junctor and feedingcircuit must be comparatively long so as to be able to perform all thefunctions involved in the exchange of information between the junctorand feeding circuit and the connected calling and called peripheralunits, such as subscriber stations.

The transmission of signal tones (e.g., dial tone, busy tone or ringbacksignal) to the peripheral units, such as subscriber stations, is alsoaccomplished during the scan period of the junctor and feeding circuitbeing used for the connection in question. The appropriate tone isconnected by linking the tone generator via appropriate tone crosspointsto a junctor circuit repeater. Hence, tone-connection and speech pathcrosspoints are provided. This prior art arrangement is exemplified byWest German Auslegeschriften Nos. 1,079,685 and 2,111,787.

West German Auslegeschrift No. 1,079,685 describes a method of using acentral tone generator whose tone signals are connected viatone-connecting crosspoints to a subscriber station; the tone-connectingcrosspoints are opened and closed in synchronism with the systemclock-pulse pattern for the tone (e.g., busy tone) to be transmitted. Itcan also be seen from West German Pat. No. 2,111,787 (VPA 71/6033, filedMar. 11, 1971) that the tone signal is connected via a tone connectionseparated from the speech path connection to a repeater winding of thejunctor and feeding circuit. In this patent, it is pointed out that thecontrol circuit for the crosspoints of the tone connection is separatedfrom the access circuit of the tone signal. It is likewise known in thetime division multiplex (TDM) switching art to connect the tone duringthe sampling period for the junctor and feeding circuit.

The principal disadvantages of the known arrangements, such as thosedescribed in U.S. Pat. Nos. 3,904,831 and 3,943,297, are as follows:

a. Greater complexity as a result of the two-wire switching in theswitching network, particularly with regard to switching control, andsupervisory means as well as greater complexity of I/O units and junctorand feeding circuits.

b. Since each sampling of peripheral units occurs only during the normalperiodic sequence of the scanning cycle, either the quantity ofinformation to be exchanged during each scan or the number of I/Odevices that can be connected, as well as of the junctor and feedingcircuits, is considerably reduced because of the long period requiredfor scanning these circuits.

c. Considerable technical effort is needed to permit the exchange ofinformation, via the junctor and feeding circuits, with the unitsconnected thereto while they are being scanned.

d. This is all the more difficult, since access is possible only via theswitching matrix by scanning the horizontal lines of the switchingmatrix.

e. Since tone connection via the tone switching matrix is likewise afunction of the scan period of the junctor and feeding circuits, thismeans an additional load for the outlay required for establishing theconnection during the scan period of a junctor and feeding circuit.

It is an object of this invention to provide without modification forthe system described hereinabove a crosspoint matrix facilitating asingle-wire connection of eight subscriber stations with four repeatersor holding circuits using an existing crosspoint matrix for two-wireswitching permitting the two-wire connection of four subscriber stationswith four repeaters or holding circuits in such a manner that withoutmodifying the existing matrix and with a minimum number of supplementarymeans.

SUMMARY OF THE INVENTION

The foregoing and other objects are achieved in that a one wire speechswitching path and a one wire tone switching matrix (8×4×1) areassembled by changing two wire crosspoint switch arrangements, includinga matrix with four rows and four columns using a two thyristor assembly(4×4×2) comprising a common row select-line and a common columnselect-line as thyristor-control lines and different through-connectingpaths.

In this way, a conventional 4×4×2 switching network matrix can beutilized, the attenuation requirements being met by simple wiring of theexisting module as an 8×4×1 switching network matrix. Thus, the existingmodule is given a broader area of application, leading to greatereconomies.

BRIEF DESCRIPTION OF DRAWINGS

The principles of the invention are described hereinbelow with referenceto the following description of a preferred embodiment given inconjunction with the drawings which are briefly described as follows:

FIG. 1 is a schematic block diagram of a PABX system in which theswitching arrangement of the invention is utilized;

FIG. 2 is a schematic diagram of a known 4×4×2 switching matrixillustrating crosspoint switching activation in a matrix of this form;

FIG. 3 is a schematic diagram of a crosspoint switching module used withthe FIG. 2 switching matrix;

FIG. 4 is a schematic diagram illustrating an 8×4×1 switching matrixachieved through a rearrangement of the 4×4×2 matrix and by using theFIG. 3 switching modules;

FIG. 5 is a schematic diagram of an 8×4×1 switching matrix whereincrosspoint connections are completed through separate activation ofthyristors in a thyristor switching module;

FIG. 6 is a schematic diagram illustrating the connection arrangement ofwires a1, s1, and t1 (FIG. 1) to a link control circuit (e.g., J2) inthe FIG. 1 embodiment;

FIG. 7 is a detailed schematic diagram of a trunk line circuit (e.g.,TLU) used in the FIG. 1 embodiment;

FIG. 8 is a schematic block view illustrating the connected relationshipbetween the customer and program memories in the FIG. 1 embodiment;

FIG. 9 is a detailed schematic block diagram of the circuitry forselecting customer memory locations for carrying out read, write anderase operations in connection with the customer memory in the FIG. 1embodiment;

FIG. 10 is a detailed schematic diagram of apparatus for applying datasignals to the storage locations M1-8 of the customer memory in the FIG.9 embodiment;

FIG. 11 is an extension of the circuitry illustrated in FIG. 10;

FIG. 12 is a detailed schematic diagram of the peripheral conversion andstorage device (PC) in the FIG. 1 embodiment and of its connections tothe faster rise time pulse transmitting bus line group (HSB) and thelower rise time pulse transmitting bus line group (LSB) therein; and

FIG. 13 is a detailed schematic view of the attendant's station (AC) andconnecting circuitry (ALC) in the FIG. 1 embodiment.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The invention will now be described with reference to a preferredembodiment of a program-controlled PABX which is to be assembled fromconventional electronic components generally employed in data processingsystems and in particular in centrally controlled telecommunicationsswitching systems. However, certain of these commercially availablecomponents must be modified for use and assembly into the inventivePABX, and where necessary the components are described in detailhereinbelow.

System Description

The PABX described hereinbelow and illustrated in FIG. 1 operatesaccording to the principles of a system wherein the subscriber linecircuits (SLCl to SLCn), the tie line circuits, interexchange truckrepeaters and interoffice trunk repeaters such as trunk line circuitsTLU, tone-frequency signal receivers R-Tf and connecting circuits ALC ofan attendant's station AC are scanned in a periodic sequence so as todetermine, on the basis of the last-look principle, if the lastoperating condition has changed. The last-mentioned four types ofcircuits or repeaters, as the case may be, and link control circuits J1to Jm, tone generators TG1 to TGx, and the logic control unit forservice SVS are likewise interrogated during idle periods, i.e., duringtime intervals between two scan periods per address code. This isnecessary in order to receive data or transmit instructions to any oneof the seven types of circuits mentioned hereinabove.

It is obvious that the one-at-a-time principle always applies to thereception and transmission of data and instructions and to the scanningof circuits. The circuit to be scanned is selected by a data processingunit CPU on the basis of a conventional program stored in a programmemory PM, which is a constituent part of the central program controlCC. In one scan period all peripheral units and other scan devices arescanned one after another. These scan periods are so chosen that thereare free times after each period. In these free times of each period ina scan cycle each of the peripheral units and also memory spaces will becontrolled by sending the address of that unit or space via the data busline, e.g., da; this is called "polling" a unit or line or memory space.

If more idle time exists in scanning period as needed, particularlyduring light-traffic periods, these idle times can be used forperforming service and test procedures. This is discussed in greaterdetail hereinbelow.

Switching network SM of the system may consist of one or more stages. Inthe present example, it is assumed that the switching network forswitching the speech paths to the subscriber line circuits, theinterexchange trunk repeaters and the attendant's station as well as tothe tone-frequency receivers is single-stage. This means that twocrosspoints in a column must be operated in the known manner to connecttwo subscriber stations together or one subscriber to one interexchangetrunk repeater. In the system under consideration, the switching networkfor switching the speech path and the speech path itself have one wire.Thyristors are employed as crosspoint switching elements. Thesethyristor crosspoint elements are operated in the call condition duringthe scan periods of the subscriber stations or of the holding circuits,whereby the latter are not scanned in a periodic sequence, but polled.

The crosspoint switches are switched on and off by polling by thecrosspoint switch used for connecting a subscriber line circuit to aholding circuit via bus line LSB/dt. The switching elements used in thesystem and the control principle as well as the various basic functionswill be described hereinbelow.

The central control unit CC of the system comprises a central dataprocessing unit CPU of conventional construction which controls allnecessary process in the system, particularly those for scanning andpolling the peripheral units with a view to receiving data and theprocessing thereof as well as the dispatch and transmission ofinstructions.

To handle these processes, the central data processing unit CPU isprovided with a scratch pad memory SPM with variable content. This meansthat this scratch pad memory SPM is always kept current on the laststatus of the data with regard to the existing conditions of thesubscriber line circuits, line repeaters, crosspoints, etc. Furthermore,the central data processing unit CPU is provided with a program memoryPM, in which the provided programs are stored, and with a customermemory CM, which at least contains the specific data for thesubscribers, various repeaters and other peripheral units and circuits.If necessary, this memory CM may be polled by the central dataprocessing unit CPU. The customer data memory CM is variable, i.e., thedata for subscribers to be stored and other peripheral units andcircuits are input by the maintenance man through selection via aperipheral maintenance circuit MC. These are known circuits and notrelevant to this invention so that they are not described in detailherein.

The transmission of data and instructions as well as addresses to orfrom the central data processing unit CPU, program memory PM, customerdata memory CM, scratch pad storage SPM and test circuit TC can beaccomplished in the bus line HSB which carry faster rise time pulses.The interrogation of the centralized units and decentralized circuits iseffected via the faster bus lines for addresses HSB/da'. The speed ofthe data transmitting on both kinds of bus lines is equal. The fasterrise time pulses have a faster rise time of the leading edge because, inthe input circuits of the HSB bus line, TTL components are used. In theLSB bus lines MOS components are used; therefore, we have in this caselower rise time pulses. The advantage is that in this case the neededperformance is lower as are the reflections if there are long connectinglines.

With regard to the transmission of data and addresses or the receptionof data from the peripheral and other units, it must be stated thatthese operations are done by lower rise time pulses as in the centralcontrol unit CC. Hence, the peripheral conversion and storage device PCis provided, which establishes the connection between the faster risetime pulses transmission circuit HSB and the lower rise time pulsestransmitting bus lines LSB. The construction and operation of these buslines, units and circuits are described in detail hereinbelow:

Dial Pulsing and Voice-Frequency Code Dialing

The PABX system described hereinabove employs telephones with decimalpulsing, as well as those using voice-frequency code dialing. To be ableto distinguish the subscriber stations using different modes of dialingfrom each other, those stations are identified with a special bit codein the customer memory CM.

The subscriber stations using pushbutton dialing all have a tone signalgenerator so as to be able to send voice-frequency dial signals. In thepresent example each number dialed consists of two out of seven or eightfrequencies. In each case, regardless of whether dial pulsing orvoice-frequency dialing is provided, the first digit of a code dialed ispassed to central data processing unit CPU, which can compare this digitinformation with the data stored in the customer memory CM so as to findout what type of signal is needed to extend the call to the wantedtelephone or interexchange trunk.

Conventional AND/OR elements are employed as comparator and evaluatorcircuits. If the interexchange trunk repeater gives access to a systembased on dial pulsing, the selected digit must be converted. This occursin the occupied interexchange trunk repeater occupied. In the case ofinternal calls, no conversion takes place, but the coded signalsdispatched from the subscriber station are passed as voice-frequencysignals directly from the tone generator of the subscriber station viathe speech path and the appropriately operated crosspoints of the speechpath network to the voice-frequency signaling receiver R-Tf and thecentral control unit. The transmission is accomplished in an appropriatesignal code of the central data processing unit CPU of the centralcontrol unit CC via the data bus lines LSB/dr if information for themarking of the crosspoint is complete.

As mentioned earlier, if necessary the two frequencies supplied from asubscriber station using voice-frequency dialing are converted in theinterexchange trunk repeater occupied, but firstly the first code digitwill be converted in the voice-frequency signaling receiver R-Tf andthen passed to central data processing unit CPU for evaluation purposes,then for an external connection the trunk repeater will be seized. Thelatter retransmits the further dialed information via the correspondinginterexchange trunk line. If necessary, the voice-frequency signals areconverted in the interexchange trunk repeater, e.g., into decimalpulsing signals, which are passed on via the speech wires in the knownmanner. These pulses are rerouted in the system frequency, i.e., as afunction of the system timing device.

This type of conversion of two-frequency signals into decimal dialpulses is effected in a manner as is known for telephone systems withboth types of dialing options.

Program Control Unit PM, Data Processing Unit CPU, Scanning, Polling

The following description illustrates the instructions defined in aprogram for execution by the central processing unit CPU. Theseinstructions control all functions in the peripheral switching units.The peripheral switching units include, for instance, the subscriberline circuits SLC1 to SLCn, the interexchange trunk repeaters TLU, thevoice-frequency signal receivers R-Tf, connecting circuit ALC forattendant's station AC, as well as the tone generators or toneconverters TG1 to TGx and the crosspoints of switching network SM andthe associated link control circuits J1 to J(m) provided in each column.A specific program listing is not given herein, but it can be readilyderived from the functions and operating sequences describedhereinbelow.

As mentioned hereinabove, the central processing unit CPU transmits anumber of signals to the peripheral units such as specific clocksignals, intermediate information, addresses and class-of-servicesignals. All these types of data are available in the semipermanentmemory SPM and customer data memory CM. The central data processing unitCPU receives, at a rate of 80 times a second, the status of eachsubscriber station (e.g., S1), of each interexchange trunk repeater(e.g., TLU), of each voice-frequency signal receiver R-Tf and of eachconnecting circuit ALC of an attendant's station AC. This means thateach unit is scanned 80 times a second, i.e., about once every 12.5milliseconds, so that, after data transmission, the rest of the 12.5milliseconds can be used for other functions as are describedhereinbelow.

When the status of one of the above mentioned peripheral units andcircuits has changed in relation to the last status ("last look"), thisinformation is received in the working memory together with the addressof the peripheral unit or circuit. During the next idle period of thescanning cycle the central data processing unit CPU can be interrogatedby polling the particular unit or circuit concerned to identify the datastored and dispatch the necessary instructions. Data are received fromthe central data processing unit CPU via the bus lines LSB/dr, LSB/dtand the peripheral conversion and storage device PC. If an excessivenumber of peripheral units or circuits or crosspoints, as well asholding circuits, wait for service by the central processing unit, andthe free time in one period of the scanning cycle is not long enough forhandling all requests stored, the rest of the processing is done in thenext idle time of the first following period of the scanning cycle. Thecentral data processing unit in such a case continues its pollingoperation for the unit waiting for service in a sequence defined by theprogram in the program storage PM. This sequence is based on apredetermined definition, using the well known criteria, of thepriorities or class-of-service and type of incoming data resulting, forexample, from the changes in status of a peripheral unit or circuit.

The following description demonstrates, as mentioned in the precedingparagraph, that the described PABX distinguishes between the normalscanning cycle and the polling of peripheral units and circuits whoseaddresses are available together with the data in working memory SPM.The central data processing unit CPU must operate on the data of theparticular peripheral unit or circuit concerned in order to be able toexecute instructions for the following operations, e.g., theestablishment of a connection for that unit or circuit. This may be theconnecting of the calling station Sn via subscriber line circuit SLCnand the speech path network SM to the interexchange trunk TL for thedesired route. The connection via the speech path network SM isestablished by operating two crosspoints in a column, e.g., k2 and k3 inFIG. 1. The speech path network SM in the PABX being described has onestage, but a greater number of stages may be used.

The polling of peripheral units and circuits by transmitting the addressof this unit or circuit via the wires dt of the data bus lines LSB inthe free time intervals of sequence periods in the scanning cycle is acharacteristic feature of this system. The advantage of this techniquelies in the fact that the entire system can be run with a singlemicrocomputer of known construction. In the case under discussion thecomputer is the central data processing unit CPU, which processes theincoming data and provides the instructions. Thus, a minimum number ofadditional modules and circuits are required. These advantages arefacilitated by insuring that the scanning and interrogation operationsoccur within the time pattern of the clock pulse generator SC.

Another important factor for the PABX is the single-wire speech path tobe switched via the speech path network SM. This single-wire speech pathis switched, preferably, by means of thyristors. Such speech pathswitching has many advantages for the system described and used herein,as will become apparent from the following description.

From the above it follows that the different time periods for thescanning and polling may be in either the same or in different timeperiods in the scan cycles. The central processing unit CPU controlsthese operations so that initially the scanning and then the pollingwill be done in the same time period of the scan cycle.

Speech-path and Tone Crosspoint Switches

As mentioned earlier, voice communication between two subscriberstations or between one subscriber station and an interexchange trunkrepeater is established by means of the switching speech path matrix SMby operating two of the crosspoint switches in a column. If, inaddition, transmitted pulsing signals must be converted, a thirdcrosspoint switch in the same column must be operated with a view toconnecting an appropriate voice-frequency signaling receiver T-Rf. Thesame is true if, for example, the exchange operator must cut in, inwhich case the particular connection can be established by additionaloperation of the contact located in the column which has been occupiedfor the connection. Such interconnection of telephones, interexchangetrunk repeaters, receivers and operator's positions by operating two ormore crosspoints disposed in a column coordinate is known and is found,for instance, in time division multiplex systems by simultaneouslyclosing two or more switches having the same pulse position. Sucharrangements are known, as well, in older PABXs.

The technique of applying tone signals by means of a special tone signalswitching matrix is exemplified in West German Pat. No. 2,111,787, asmentioned hereinabove. However, in the invention described herein aspecial feature of the tone connection mode lies in the particular wayof connecting the tone by using single-wire switching matrices, i.e., asingle-wire speech path switching matrix as well as a single-wire toneswitching matrix. The special nature of this tone-signal connection tothe speech path is essentially formed by the control leads for thecrosspoint which is to be through-connected.

The general system description shows that the essential current riseneeded for through-connecting via the crosspoints is brought about bythe current sink for the speech path crosspoints in the subscriber linecircuits or interexchange trunk repeaters and for the tone-connectioncrosspoints in the link control circuits. The through-connection issupplied from a link control circuit J1 to Jm for the speech pathcrosspoints as well as for the tone crosspoints. Each link controlcircuit is assigned to a column of the crosspoints of the speech pathswitching matrix and to a row of the tone signal switching matrix.However, one separate output is provided in the relevant repeater toachieve this.

The tone generators TG1 to TGx convert the square wave pulses receivedfrom the central clock-pulse generator SC into sinusoidal signals madeup of one or more frequencies.

In order to though-connect the crosspoints, the subscriber linecircuits, the interexchange trunk repeaters, the connecting circuit,etc. as well as the link control circuits and tone generators, arepolled from the central data processing unit via the address linesduring the idle times of scan cycle periods. To disable the crosspoints,new polling with an accompanying disabling instruction is necessary.This is also essential with a view to the tone-signal transmission sincein this case, in addition to the tone-signal identification, one mustindicate what clock pulse is involved to enable the tone to be removedin conformity with the ON time and OFF time.

FIG. 1 shows that via line t1, which represents the cathode line for thetone crosspoints tk1 and tk2 disposed in a column coordinate and whichis polled via the link control circuit J2 (current sink) the connectionof the tone to anode line a1 of speech path crosspoints k4 to k7,disposed in a column coordinate occurs via a bypass capacitor (e.g., C).If, for example, one views the connection of a tone signal from the tonegenerator or tone converter TG1 to the line connected to subscriber S1via the crosspoint k4, this connection requires operation of the tonecrosspoint tk1.

To achieve this, the cathode line t1 must be accessed from the linkcontrol circuit J2, the anode line ag1 from tone generator TG1, and thecontrol electrode from the tone generator TG1 via the control wire s1.The accessing occurs by polling the tone generator TG1 and the holdingcircuit via the address lines "da" of the data transmitting bus line LSBand the address output unit AX of the peripheral conversion unit PC. Thelatter unit and, thus, the address output unit AX receives the addressesof the tone generator to be accessed and of the repeater to be accessedfrom working memory SPM under the control of the data processing unitCPU. The working memory SPM receives its data allocated to individualconnections from the customer memory CM, in which signal-connection anddelay signals for each tone generator are stored. It is there indicatedwhich tone signal must be transmitted to which subscriber and which linkcontrol circuit is used. The activation of the crosspoint switches iseffected by polling in the some manner as the corresponding transmittingof information.

The information as to which tone signal must be transmitted yields aninstruction for the tone connection and removal as a function of acentral clock generator SC. This means that the tone crosspoint isclosed and opened as a function of connection and removal instructions.A tone generator is assigned as a tone converter to each clock pulse.

As will be described hereinbelow, the data in the customer memory may bealtered from a maintenance position via a maintenance circuit MC and thebus lines LSB/HSB by keying or dialing in, so that clock pulses assignedto a tone generator, too, can be varied.

In connection with this application of the tone signal, there must beassurance that only one tone signal at a time will be applied to aspecific connection. Double connections are not possible without faultyactivation of crosspoints in the circuit arrangement defined herein.

Alteration of Clock Pulses for Existing Signals or Initiation of NewSignals

The clock pulses of the signals to be transmitted are determined byappropriate control of the tone-connection crosspoint switches, andthrough-connection, as well as separation of the crosspoint switches, isdependent on the polling of each crosspoint and one of the correspondingtransmissions of instruction signals. The clock pulses associated with agiven tone signal, e.g., calling signal, busy signal, etc. are stored inthe program memory PM and are polled with each activation of atone-connection crosspoint for the first (e.g., "ringing tone")connection during a call to be established as a function of the ringingtone connection instruction from the central data processing unit. Inaddition to this timing information, i.e., "calling signal" the workingmemory SPM will store the further call data, such as the telephoneaddress and the tone generator address. The timing information containsdata on the initiation and end of the clock pulse and on the differentforms of signal, i.e., the purpose of the signal, such as ringing, forexample.

In order to have the option of altering the clock pulses (preprogrammedin the system) of specified signals (e.g., ringing) as well asinitiating new tone signals, the program memory PM must either beprogrammable in a simple manner or the timing of specified signals mustbe stored in the customer memory CM assigned to the tone generatorsinstead of in the program memory and have facilities for interrogationthere. In any event the customer memory CM is available as a memory thatcan be altered at any time, i.e., a random access memory (RAM) is used.

It should be noted that each of the tone generators is allocatedaccording to the different frequencies and not to a specific signal(e.g., ring signal). The connection of special devices, such as taperecorders and also other data devices, may also be effected over one ormore special tone generators in the speech path.

Dictating machines, television sets for conferences, data terminals,etc. can be connected with the speech and/or special matrix like theother peripheral units. The assignment of tone generators containingconverters for specific clock pulses simplifies the control of thetone-signal crosspoints and will lead to savings in memory capacity. Thecentral clock pulse generator SC is employed to synchronize theinstructions dispatched from the central data processing unit CPU withthe clock pulse available in the system.

Control of Crosspoint Switches in the Case of Single-Wire Switching

Thyristors are preferably employed as crosspoint switches in the systemdescribed herein, and these are combined in a switching network to forma switching matrix in the manner to be described below. This switchingmatrix is a 4×4×2 crosspoint switch, i.e., a matrix having 4×4dielectrically insulated thyristor modules disposed in four rows andfour columns and with two thyristors per crosspoint. Thus, two-wireswitching of speech paths is possible (FIGS. 2 and 4).

If in the described system the same switching matrix is employed forsingle-wire switching, it must be modified appropriately so as toactivate the crosspoints, assuming the use of the same crosspointswitching devices. Thus, the switching matrix is to be modified to forman arrangement with eight rows and four columns and one crosspointswitch per crosspoint, i.e., with 8×4 thyristors with one crosspointeach, or 8×4×1.

FIG. 2 illustrates the activation of the crosspoint switches in theknown 4×4×2 matrix. The column in which the crosspoints to be activatedare disposed is determined over one of the two control leads 1 or 2 incolumns A, B, C or D, and the row is determined over one of the controlleads 1 or 2 in rows W, X, Y or Z. Both thyristors disposed at acrossing point are through-connected, not only in the presence of thecorresponding control signals on the control leads for the determinationof the row and column, but also only after previously turning on theappropriate cathode and anode potentials.

FIG. 3 illustrates the construction of a thyristor module forming acrosspoint switch.

In order to obtain a 8×4×1 matrix for the telephone switching systemdescribed herein, the existing 4×4×2 matrix must be rearranged. Toachieve this, the thyristors through-connected via the anode A1 (FIG. 3)are followed by those disposed in a column coordinate andthrough-connected via the anode A2. This is shown by a dotted line inFIG. 4 between the thyristors Th1 and Th5. At the same time, and alsoconnected in parallel with the cathode inputs 15 and 22 of thethyristors Th1 and Th5, a connection to the control lead W isestablished via the diodes WD1 and WD2. This control lead W is intendedfor selecting the row in which a crosspoint is to be operated. This isto assure that only the thyristor which is determined via a cathodeinput (e.g., 15) and the input (e.g., W) of the control lead, as well asvia the vertical control lead (e.g., A) can be activated orthrough-connected. To determine the relevant column, the input A, B, Cor D is provided without change.

FIG. 5 shows an arrangement for a switching matrix which does notrequire correlation between two thyristors and a thyristor module. Itshows that by connecting in parallel the thyristors associated with acolumn, coupled with the possibility of separately activating the twothyristors associated with a given thyristor module, the existing 4×4×2matrix can be converted into a 8×4×1 matrix.

To operate a crosspoint switch in this manner, the following processes,for example, must be executed: mark a subscriber station (e.g., S1),thereby marking the subscriber line circuit and turning on the cathodepotential, e.g., across 22 (the current-sink circuit remains energizedeven after closing until opened by a separation instruction); determineat the same time the row, then mark a holding circuit (e.g., J2) byinterrogating it and connecting a control signal (e.g., to line E1), andat the same time determine the column. In this way, eachthrough-connection in the switching matrix occurs when the anodepotential (e.g., across A1) is turned on (FIG. 1).

It is apparent from the above that the connection of the cathode linesW1, W2 via the diodes WD1, WD2 with the control lead W permitssingle-wire switching via the particular matrix. The diodes WD1 and WD2prevent crosstalk.

Link Control Circuit

By means of one of the link control circuits (e.g., J1 in FIG. 1) thereoccurs the activation of the speech-path crosspoint switches as well asthe call setup from the anode line to the cathode line via athrough-connected thyristor and the tone-signal connection via anadditional (likewise through-connected) tone crosspoint switch and itscathode line (as shown in FIG. 1). In addition, the link control circuitfacilitates an appropriate regulation of attenuation and when aspeech-path crosspoint switch is operated, it signals the correspondingmodule.

FIG. 6 shows the scheme for connecting the wires a1, s1 and t1 to thelink control circuit (e.g., J2). The current sink of the link controlcircuit is labeled CS. As shown, the wire t1 is connected to the base ofthe tone-crosspoint thyristor TK1. The control wire for the speech-pathcrosspoint is labeled g. The point JA2 is the scanning point for thecrosspoint; that is, it is the point activated via the interexchangetrunk and PBX-power-lead repeater so as to through-connect or disablethe particular crosspoint concerned. In the control connection to thepoint JA2 there is likewise connected a photo coupler CR intended forsignaling the operating condition to a module when the crosspoint isactivated. A loss pad TK3/TK5 is connected to the anode line a1 of thespeech path crosspoint. This pad TK3/TK5, which is or is not activatedby the common control unit CC as a function of the form (i.e., short orlong) of the connected interexchange trunk, is connected with anode linea1 of the speech path crosspoint. This pad is needed whenever shortlines are connected. In the present instance, the attenuator circuitcauses an interposition of an additional resistance of about 300 ohm.

A capacitance CE forms a connection to the anode line over which thetone signal is linked to the speech path (C in FIG. 1). It should benoted that the anode line is connected to all speech-path crosspointsdisposed in a column coordinate and that in order to establish voicecommunication at least two such crosspoints must be operated. Hence, itfollows that, depending on the number of operated crosspoints of one ormore subscribers, a signal can be transmitted via the capacitor andtone-signal connecting point, for example not only to the calling orcalled party, but also to all parties involved in a call in progress.

It should also be noted that the activation of the loss pad occurs viathe wire EV2 by interrogation via the slower bus line LSB.

As mentioned earlier, the crosspoint switch of the tone switchingnetwork is operated in synchronism with the prescribed clock pulsepattern. This is done through activation via the control wire 1 byapplying appropriate connection and removal signals from the centraldata processing unit CPU over the data transmission circuit.

The Trunk Line Circuit (FIG. 7)

The following is a description of the functions of a trunk line circuit,such as the circuit TLU in FIG. 1. In the trunk line circuit seizure maybe effected from the trunk line, i.e., from the general switchedtelephone network both with a "ground start" and with a "loop start".

If in the idle condition no potential whatever is applied to the tipconductor, an incoming seizure occurs by connecting ground potential("ground start"). By connecting this ground potential in the case of anoutgoing seizure, the acknowledgement signal is received from theexchange side. In the case of the "ground start", a negative potential(-48 volt) is continuously applied across the ring connector. In thecase of a "loop start", ground potential is continuously applied fromthe exchange side to the tip conductor, while -48 volt is continuouslyapplied to the ring conductor. In such a case, seizure can only occur bytransmitting an alternating current ringing signal. The processes inconnection with both modes of seizures will be described hereinbelow.Since it must be possible to connect an outside repeater to generalswitched telephone networks with the first mode (ground start) as wellas with the second mode (loop start), the trunk line circuit must alwaysbe connected in such a manner that both seizing modes are equallyfeasible.

In the outside repeater shown on the right-hand side, both speech wiresT and R (tip and ring) are connected to the trunk line and, therefore,to the general switched telephone network while the wires shown on theleft-hand side represent the address circuits, the data circuits, aswell as one of the two speech wires. This speech wire is marked g. Theother wires, particularly the signaling wires 1-6, are connected to thedata transmission circuit group LSB at a lower data signaling rate,namely, for the data, address and information exchange with the centraldata processing unit CPU. The exchange side of the outside repeater isdesignated as the secondary side of a transformer LU and must besuitable for any exchange-side connecting and ringing mode.

Trunk Line Circuit With Ground Start

In the idle condition, the transistor T1 is switched on via thesignaling wire 4 due to the potential applied and remains energized viaa conventional holding circuit not shown in detail. Upon scanning thetrunk line circuit, its idle condition is recognized because of theexistence of the holding circuit.

If the incoming seizure in the case of a ground start occurs throughapplication of ground potential to the tip conductor, a loop in thetrunk line circuit is activated via the ring conductor and is connectedfrom the top connector via the bridge rectifier G, not only directly viathe photo coupler L1 to the ring conductor, and via additional circuitsconnected in parallel in which are disposed, inter alia, a diode D1, aresistor R1 or the transistors T2-T5. The resistor R2 and a pair ofdiodes D2 are likewise connected in one of these additional circuits.Due to the activation of the photo coupler L1/T6, the transistor T6,which is under the influence of the photo coupler, is activated via the+12V potential and resistors R3 and R4, as well as via resistors R5, R6,R7, and the amplifier V1 and control lead 7 are then activated. Parallelcircuits still exist via the capacitor C1 and the resistor R8. The trunkline circuit is indicated as occupied by the data processing unit CPUvia wire 7. Transistors T1 and T6 are conductive. This is valid as anindication that a seizure with ground potential has taken place (groundstart).

If an alternating current ringing signal is simultaneously transmittedfrom the general switched telephone network upon application of groundpotential to the tip conductor, the transistor T7 in the circuit is alsoswitched on via the resistors R9, R10, R11 and the capacitors C2, C3, aswell as the diodes D3-D5. The detection of the ringing signal in thetrunk line circuit is necessary, since the disconnection of the ringingsignal, when the called party in the PABX answers, must be controlled bythe trunk line circuit. Upon simultaneous transmission of groundpotential via the tip conductor and the ringing signal, the evaluationof the ground potential for seizing purposes is likewise effected by theevaluator circuit via the photo coupler L1/T6, thereby activatingtransistor T6 in the manner specified hereinabove.

When the called party answers, this is detected by the data processingunit CPU upon scanning the outside repeater. When the called partyanswers, the transistor T8, as well as the relay K4, and activated viathe wire 1 and the gate G1. Moreover, the transistor T12 has beenswitched on via the wire 6. As a result, photo coupler L3/T9 isactivated and thus, the transistor T9, which is connected to theresistor R12 and the capacitor C4 in bridge rectifier G, is switched on.This causes the direct current flowing through the tip and ringconductors to rise more than tenfold (e.g., from 2 mA to 26-35 mA). Thisdc current rise is detected in the general switched telephone network asanswering, causing the ringing signal to be disconnected.

During the conversation, the above mentioned transistors T9, T2, T3, T4and T5, as well as T1, T8 and T6 remain conductive.

To release the connection initiated by the subscriber to the generalswitched telephone network the potentials are turned off from the ringand tip conductors. However, the free indication occurs with a time bagso as to prevent an immediate new seizure and to first ensure aninternal full release of all switching elements seized and operated.Subsequently, only the circuit is switched through via the transistor T1to permit new seizure.

If the connection being established is an outgoing seizure from thetrunk line circuit, the transistor T10 is switched on via the wire 5,and the relay K3 is activated upon detection of the off-hook conditionof a calling station and concurrent detection of the idle condition ofthe trunk line circuit after the trunk code has been selected. Thecurrent-sink circuit CS is connected to render the ring conductoroperative through contact 1k3 with the result that the current for thering conductor rises to about 50 mA. At the same time, a potential isapplied to the tip conductor by switching the transfer contact 2k3, thatis, a loop is established via the photo coupler L1 (T6) and therectifier bridge B between tip and ring conductors. If the line repeaterof the general switched telephone network has detected the increase incurrent to the ring conductor as a seizure, ground potential will beapplied to the tip conductor as an acknowledgement signal, activatingthe photo coupler L1/T6, so that transistor T6 is again renderedconductive. As a result of this circuit condition (i.e., conductingtransistors T1, T6 and T10), a ringing tone is also transmitted to thecalling party so as to indicate the seizure of the general switchedtelephone network. The dial signals now selected by the subscriberstation of the PABX are transmitted via the wire 3 to the photo couplerL3/T9 and from there to the transistor T9, so that appropriate increasesin potential corresponding to the transmitted dial signals are sent tothe tip conductor. These potential increases are passed to the generalswitched telephone network and further evaluated there.

In the case described hereinabove, the photo coupler L1/T6 and, hence,also the transistor T6, are controlled during the transmission of thedial signals with the result that the potential applied to the wire 7would also vary continually. However, in the present case the wire 7must remain on a sustaining potential, and the diode D is provided toachieve this. With the diode D6 the drawing of continuous current at theend of each dial pulse from the input of the gate G2 coming from thewire 3 is assured. In this way, the unwanted potential change to thewire 7 is avoided.

Upon completion of the call, the connection is released, as describedhereinabove

Trunk Line Circuit With Loop Start

If, instead of the seizure of the PABX from the general switchedtelephone network with application of ground potential of the tipconnector ("ground start"), the seizure from the telephone network iseffected with a "loop start", ground potential is continuously appliedto the tip conductor, in idle condition, and a negative potential to thering conductor. The incoming seizure from the telephone network occursexclusively through the application of signaling alternating current ofthe ringing signal. In this case, a current rise occurs in the circuitthat detects the application of ground potential in the case of a groundstart with the result that, in addition to the photo coupler L1/T6 andthe transistors T2, T3, T4, T5 and T6, the transistor T7 is alsoswitched on. The resistor R7 therefore will be lower as for the groundstart condition. The increase in current caused by the ringing signalinitiates with one half-wave the activation of the photo coupler L1/T6and with the other half-wave that of the photo coupler L4/T13,alternately controlling the transistors T6 and T13. Thus, an outputsignal is also applied alternately to the wires 7 and 8. It should benoted that the circuit via the transistor T7 represents a controlledfunction for the photo coupler L1/T6, ensuring its satisfactoryresponse.

As explained earlier, answering the subscriber of the PABX causes thephoto coupler L3/T9 and the relay K4 to be activated. A speech circuitis established by means of contact K4 via the secondary winding oftransformer U, and transistor T9 and hence, also T11, are switched onvia photo coupler L3/T9. As described earlier, the direct current isincreased more than tenfold via the speech wires. This involves theactivation of the current sink. In the general switched telephonenetwork this is detected as an answering by the called party and theringing signal is cut off. The release occurs after establishment of thecall condition, as explained above.

If seizure of the trunk line is effected from the calling party, whendata processing unit CPU detects the removal of the receiver by thecalling party during a scanning cycle, the signal is disconnected fromthe wire 4 and, instead, a potential is applied to the wire 7. Thiscauses the transistor T12 to be switched on, thereby activating thephoto coupler L3/T9 and the transistor T9. As a result, the currentthrough the speech wires is increased, which increase is interpreted inthe general switched telephone network as a seizing signal. This networkdoes not return an acknolwedgement signal. The increase in current actson the photo coupler T1/T6, with the result that a signal is applied tothe wire 7. This is detected by the data processing unit CPU during thescanning of the outside repeater. After detection of the seizing signalin the general switched telephone network, a dial tone is sent to thePABX. As described above, the transmission of the dial signals, e.g., DCdial signals, occurs by generating current rises in the speech wires viathe transistor T12 and the photo coupler L3/T9. Release follows afterthe connection has been established, as described above.

The Customer Memory CM (FIGS. 1, 8, 9)

The customer memory comprises a storage that can be altered byelectrical means (e.g., an electrically alterable read only memoryEAROM). New data are input to this customer memory CM from theperipheral unit of the maintenance circuit. The storage locations areselected via the existing faster bus lines HSB with the aid of availableaddresses which are not utilized further. Random access of the centraldata processing unit CPU to the customer memory (also for testingpurposes) is assured by using the faster bus lines HSB. Further, byutilizing, the faster bus lines, the number of addresses available forselecting storage locations is so large that data can be placed intostorage and fetched in a polling process for all subscribers who can beconnected to the system.

As shown in FIG. 8, customer memory CM is advantageously combined withthe existing program memory PM in such a way that a certain area of theprogram memory PM receives a portion with higher-level customer data,that means group addresses which, like the other storage locations ofthe program storage PM, are selected and sampled by the central dataprocessing unit CPU via the faster bus lines HSB and the decoder DPM. Onthe basis of the information available in the program memory PM, certainstorage locations are selected for the purpose of fetching specific(e.g., subscriber-specific or peripheral units) individual informationvia an additional decoder DKDS in another smaller memory KDS. Thesampling result is routed to the central data processing unit CPU viathe data output lines da of the faster bus lines HSB for the furtherinformation.

The utilization of idle storage locations of the program memory PM forcombination with the selection of storage locations of the customermemory has the advantage that the program memory can be loaded to fullcapacity, while the existing faster bus lines HSB and address repertorycan be fully utilized for the selection of the customer memory. Inaddition, this reduces the necessary size of the customer memory to abare minimum.

FIG. 9 shows details of the selection circuit for storage locations ofthe customer memory for reading, writing and erasing informationtherein.

By means of an address applied to the input of the decoder PM via theaddress bus lines HSB/da, by that address a given storage location isselected in one of the two address buffer registers AR1 and AR2, withthe result that at an output ar11 of the address register AR1 and at anoutput ar21 of address buffer register AR2 a complete information blockis applied to the inputs of storage devices M1-M8 dependent of thesimilar transmitted status and command information, which are receivedvia the data bus lines HSB/dr in separate bit series. At the same timethe information is going to the storage devices M1-M8, the storageaddress goes to the decoder HS to select the storage location to store,to read out or to erase the customer data assigned, for example, to atelephone station.

Parallel to the selection processes described above, storage locationsin an instruction register BR, a data input register WDR and a readregister RR are selected with the polling the address register. Thewriting of data into or the reading of data from one of the storagedevices M1-M8 (outputs AU1-AU9) occurs via the addressing by the decoderHS. These data items are transmitted via the data bus lines dr or theread line dt and rerouted. The erase, write or read instructions are apart of the transmitted information of the data processing unit CPU viathe data bus line HSB/dr. This information flows by way of theinstruction register BR, WDR, RR via the outputs br1, br2, br3, wdr, rrto the storage devices M1-M8. An acknowledgement signal is passed to thedata processing unit CPU via an output br4 of the instruction registerBR. It is ensured via an input brs that if a register module is notplugged in, the ongoing sequence of operations is not executed and thatthe data processing unit CPU recognizes this. The CPU will then eitheroccupy another register and continue the operation or start with a newswitching process. Signaling may occur in parallel with the foregoingoperation.

For the erasing, writing and reading operations of the customer datamemory KDS fast rising pulses are needed. This means that the storagecapacitor SpC provided in the corresponding circuit (FIG. 10) must becharged very rapidly. Current pulses are necessary to achieve this.However, the transistors are endangered by these current pulses. Toremove this risk, field effect transistors that can be turned on via acurrent sink must be employed.

A potential change from +5V to -23V and lasting, for example, for aperiod, approximately equal to or exceeding 100 ms is needed as an erasesignal. The permissible tolerance range is very narrow (e.g., ±5%).

Storage Arrangement for Customer Memory CM

FIGS. 10, 11 show the special changeover unit for the fast loading anddischarging of storage capacitor SpC. As mentioned earlier, this storagecapacitor SpC is conditional upon the fast operating time of storagedevices M1-M8.

The circuit shown in FIG. 11 is only to control the input information onthe connecting point EM1 in FIG. 10 and FIG. 11.

Dependent on the commands for erasing, writing and reading there will bea different status on the outputs br1, br2 and br3 of the instructionregister BR. For writing the output br1 will carry a sequence of 200square waved (+) pulses. The voltage alternates between +5V and -23V.That means for a period 5 us there is a +5V voltage and for a period of100 us there is a -23V voltage of the output br1. For the erasing therewill be an inactive status on br1, br2 and br3. For reading there willbe an on pulse. The voltage alternates one time from +5V to -14V (aperiod of 1 us) and back to 5V. Dependent on information from thecentral processing unit CPU transmitted via the faster data bus lineHSB/dr, the capacitor SpC must be charged and discharged dependent onthe information flowing to the storage devices M1-M8. This charging anddischarging will be allowed by changing the voltage on the connectingpoint EM1 by the reading, writing and erasing commands.

In the circuit in FIG. 10 the transistors TT5 and TT6 turn on fasterthan the transistors TT1 and TT3.

The field effect transistors TT2 and TT4 only work as current sinks toget a limited current on the transistors TT1 and TT3, to protect themfrom short circuit and they are always in an on state.

If -23V is applied to the point EM1, the capacitor C2 will be charged,and if it is charged the transistor TT6 is turned on for the 100 usperiod that the -23V is connected.

Then the transistor TT5 will be turned off, as will transistor TT1, butslower. If the transistor TT1 is turned off the transistor TT3 is turnedon, and the capacitor SpC is loaded.

These operations are done very fast. After the -23V is turned off ofEM1, the +5V voltage is turned on for a period of 5 us. The transistorTT5 is turned on. The transistor TT6 is turned off. The transistor TT1is turned on and the transistor TT3 is turned off. The time during whichtransistor TT3 is not already turned off the field effect transistor TT4protects the transistor TT3.

The capacitor SpC is discharged. These operations are repeated as oftenas the voltage on EM1 is changed.

Allocation of the Scanning Cycle

FIG. 12 shows the peripheral conversion and storage unit PC (FIG. 1)which is to act as a connecting link between the faster bus line HSB andthe slower peripheral bus line LSB. Each of the two data transmissionline groups can be subdivided depending on the purpose. They are, first,recording circuits dt1 over which is transferred the information fetchedfrom specified peripheral units and to be stored in the common controlunit CC, e.g., in connection with the charge registration or routinetesting. Second, there are outgoing and incoming data circuits dt2 ordr, over which the instructions are transferred in the outgoingdirection to the peripheral units and subscribers and, conversely, thedata transmitted from there.

Third, they are the circuits un and sv over which the peripheral unitsand subscribers must be scanned in successive cycles from the commoncontrol unit CC and, thus, from the central data processing unitrelative to its instantaneous condition.

Fourth, various trunk groups are provided for scanning the peripheralunits, over which trunk groups the particular circuit conditions ofperipheral units are interrogated for performing switching processesabout to be executed, such as call establishment or changeover (e.g., inthe case of a consultation or transfer call).

It should be noted that the bus lines communicating with the centralcontrol have as many wires as are needed for transmitting the parallelbinary code being used. If, for example, 256 addresses are needed, therewill be eight wires for carrying 8 parallel bits.

To utilize the address repertory as effectively as possible, the unitmust be so selected that it is assigned a group address, an area addressand, within the area, a unit address, for example, in order to identifythe holding circuit group, the area within the group, and the repeatersthemselves. FIG. 12 shows the corresponding address bus lines gr, se anddv.

A periodically completed scanning cycle lasts 12.5 milliseconds, if eachperipheral unit is scanned 80 times per second, as mentionedhereinabove. Since only 1 millisecond is needed for transmitting thedata to be sampled, the remaining time of 11.5 milliseconds within thescanning cycle is available for other purposes.

For the reasons set forth above, there are provided, in addition to theuniversal scanning circuits un and sc for the periodic scanning of thetelephone stations, repeaters of every kind (e.g., J1-Jm), tonegenerators (e.g., TG1-TGx), connecting circuits ALC for attendant'sstations, voice-frequency signaling receivers R-Tf, etc. There arefurther provided for the scanning of units assigned to a special testingposition for associated units in case of need or also as a matter ofroutine, special lines fa that permit a more rapid scanning of specifiedperipheral units.

More important than the above additional scanning is the fact that inthe available time of 11.5 milliseconds per period of the remainingcycle, telephone stations, units and circuits are polled by means of thecentral processing unit CPU. Thus, the central processing unit CPU gainsaccess to these peripheral units so as to be able to handle instructionsand information for processing sequences of operations waiting to beswitched. If 11.5 milliseconds are not adequate for such operations, theincomplete switching process and associated polling of the peripheralequipment are continued within the following free time in the sequenceperiod of the scanning cycle after completion of the normal scanning ofthe peripheral equipment.

The idle times in each period in the scanning cycle are also utilizedfor other purposes, for example, as mentioned above, to poll storagelocations in the customer data storage for the purpose of establishing aconnection, i.e., for reading customer-specific data necessary for thecall setup simultaneously with the selection of a peripheral unitinvolved in the call (e.g., link control circuit or interexchange linerepeater), depending on the particular call condition.

If data in the customer memory must be erased and written anew, ifcertain routine test procedures must be performed according to aprespecified program, or if maintenance operations must be controlledexternally, certain units of the system, as well as the test panelinvolved or the maintenance circuit, must also be polled. This isnormally done during low-traffic periods in the remaining idle periodsof the scan cycle. Processes not performed during an idle period of thecycle are performed in the next idle period of a subsequent scan cycle.

In the example under discussion, erasing and new writing of data in thecustomer data storage, as well as control of testing procedures, takeplace from the maintenance. To accomplish this, the data exchange withthe common control unit occurs only via the faster bus lines. Themaintenance processes are also controlled via the maintenance circuitMC.

The ringing signal generator RG for the subscriber stations supplies themaintenance circuit with the clock pulses necessary for routine tests.The ringing signal generator is powerline-operated via the line SVS.

The maintenance circuit exchanges data via the slower (LSB) and thefaster (HSB) bus lines.

Attendant's Station AC -- Attendant's Connecting Circuit ALC

FIG. 13 shows the circuitry for the transmission of every type ofinformation to the attendant's station and from there to the connectingcircuit ALC using a minimum number of transmission lines. To accomplishthis, the signals to be transmitted are sent as serial-code dc signals.FIG. 13 shows the transmitter of the attendant station marked S1. Thistransmitter applies the coded signals to the transmission path ADR.These signals consist of potential connections and disconnections ofvarious durations. The same is true for the transmission in the oppositedirection by the transmitter S2 via the transmission path ADT.

The reception of the transmitted signals and, thus, the transmission tothe evaluator means, not shown, at the receiving end occurs via thecorresponding photo couplers OT1 or OT2. Thus, the provision of eachattendant station with only one speech wire pair and four signaltransmission wires is assured.

The principles of the invention are described hereinabove by describingthe construction and operation of a system constructed accordingly. Thedescribed system is, however, to be considered only as being exemplaryof those principles. A number of modifications to or changes in thedescribed system will appear obvious to those skilled in the art, but itis to be remembered that the invention is defined by the appended claim.Thus, such modifications or changes will be within the ambit of theinvention as defined by the claim.

I claim:
 1. In an electronic telephone switching system havingperipheral units for carrying signals between subscribers, saidperipheral units including a plurality of trunk and tie line circuits,at least one attendant line circuit, a plurality of signaling tonereceivers and a plurality of link control circuits, having a pluralityof tone generators for placing signal tones on said peripheral units,having speech path and tone switching matrices connected to and operableto interconnect said peripheral units and no interconnect said tonegenerators with through-connections between said peripheral units,having central control means constituted by a plurality of componentsincluding a central processing unit, a program memory connected to saidcentral processing unit, a customer memory connected to said centralprocessing unit, and a scratch pad memory connected to said centralprocessing unit, having system timing means connected to said centralprocessing unit, and having bus lines interconnecting the components ofsaid central control means and other bus lines for connecting saidperipheral units to said central control means, the improvementcomprising:said speech path and tone switching matrices beingconstructed utilizing, respectively, a 4×4×2 crosspoint matrix; aplurality of electronic crosspoint switch means, each of said switchmeans being assigned to a respective matrix crosspoint, each of saidswitch means comprising a pair of thyristors, the control terminals ofwhich are, respectively, connected to common column and common rowselect lines, the conducting paths of said pair of thyristors beingconnected, respectively, between differing row and column lines at agiven crosspoint; and zener diodes for separate marking of each of saidthyristors in said pairs thereby avoiding crosstalk between connections.